The present invention relates to a control rod, and more particularly, to a control rod ideally applicable for a boiling water reactor to control reactor power.
The structure of conventional control rods used in a boiling water reactor and installation environments will be described. The boiling water reactor is equipped with a reactor core, which is loaded with a plurality of fuel assemblies, in the reactor pressure vessel. Uranium 235 contained in nuclear fuel material included in the fuel assemblies absorbs neutrons and generates nuclear fission, thereby generating heat. Reactor water (cooling water) supplied in the core is heated by the heat and boils, and some part of the water turns into steam. In the core, neutrons newly generated by the above-mentioned nuclear fission cause another uranium 235 to fission, thereby generating a chain reaction.
To control the amount of chain reactions of nuclear fission, control rods including neutron absorbers therein are utilized. Among those, a control rod normally used in a boiling water reactor has a crucial cross-section and is inserted into a gap (saturated water area) formed among each channel box contained in four fuel assemblies. One control rod is disposed per cell including four fuel assemblies. A control rod guide tube is disposed below the four fuel assemblies for almost each cell. The control rod guide tube is disposed in the reactor pressure vessel. One control rod uses each channel box contained in four fuel assemblies in a cell and the control rod guide tube as guide members. Furthermore, the lower end portion of the control rod is connected to a control rod drive mechanism, and the control rod is inserted into the reactor core by the operation of the control rod drive mechanism and then withdrawn from the reactor core. The control rod is an important device used for controlling reactivity and regulating power distribution.
The structure of the conventional control rod used in the boiling water reactor will be described briefly. The control rod has a handle, a tie-rod, a fall velocity limiter and four blades. The handle is welded to an upper end portion of the tie-rod. The fall velocity limiter is welded to a lower end portion of the tie-rod. The four blades extend in four directions from the tie-rod located in the central axis of the control rod. Each blade has a U-shaped sheath mounted to the tie-rod, and a plurality of neutron absorbing rods that contain neutron absorbers are disposed inside the sheath (see Japanese Patent Laid-open No. 2002-257968). Japanese Patent Laid-open No. 2002-257968 further describes that a plurality of projecting portions are formed on the end face of the sheath in the axial direction of the sheath, and those projecting portions are welded to the tie-rod by laser welding. By providing projecting portions, the sheath is intermittently welded to the tie-rod in the axial direction. Furthermore, another well-known control rod is structured such that a hafnium plate, instead of a neutron absorbing rod, is disposed in the U-shaped sheath welded to the tie-rod (see Japanese Patent Laid-open No. Hei 8 (1996)-105989 and Japanese Patent Laid-open No. 2006-153522).